Characterising the effects of high-pressure rolling on residual stress in structural steel welds

The large residual stresses which occur in welded objects are an unavoidable consequence of the non-uniform cycle of thermal strain inherent in most welding processes. Furthermore, the particular distributions of residual stress which are characteristic of welding can adversely influence several material and structural failure mechanisms, including fatigue fracture, elastic fracture and buckling. This thesis describes an experimental investigation into the use of localised high-pressure rolling of the weld seam for the purpose of residual stress reduction in steel welds. In preliminary experiments, it is demonstrated that the transient stresses which occur in an object while part of it is welded or rolled, can be inferred from strain measurements taken during the process. Furthermore, such measurements can be used to estimate the resulting residual stresses. Good agreement is observed between residual stress distributions found using this method and determined using neutron diffraction. The effect of rolling on residual stress in structural steel welds is then investigated using both of these measurement techniques. Rolling is shown to greatly reduce tensile residual stress at the weld seam, even introducing compressive stress when a greater rolling force is used. However, this is only the case when rolling is applied post-weld: by contrast, methods involving rolling prior to or during welding do not improve the residual stress distribution. It is proposed, on the basis of transient stress measurements, that this is because the deformation which occurs in a weld during cooling greatly exceeds its yield strain, and so any effect of high-temperature deformation on residual stress is subsequently erased. Other effects of rolling on the properties of a weld have also been studied. Using mechanical tests and microstructural analysis it is shown that while post-weld rolling causes work-hardening of structural steel welds, rolling the weld at high temperature results in refinement of the weld microstructure, also hardening it. The effect of roller geometry on residual stress and fatigue life of rolled specimens has been investigated: the induced residual stress distribution is relatively insensitive to the roller’s crosssectional profile, while the fatigue life is shown to be reduced by post-weld rolling. The implications of these findings for the practical implementation of weld rolling, along with many other applied aspects of the process are discussed. While rolling is undoubtedly a useful and highly effective tool for residual stress mitigation in welds, its secondary effects should always be carefully considered.